1. Field of the Invention
The present invention relates to a semiconductor element and a method of manufacturing the semiconductor element, and particularly, relates to an insulated gate bipolar transistor used in a power conversion device and to a method of manufacturing the insulated gate bipolar transistor.
2. Description of the Background Art
Amid a progressive reduction in the power consumption of a power conversion device, there is a large expectation for a reduction in the power consumption of power devices themselves used in the power conversion device. Among the power devices, the use of an insulated gate bipolar transistor (hereafter called an IGBT), wherein a low on-voltage can be achieved by a conductivity modulation effect, and gate control is possible using voltage, has been established. In recent years, the application of a trench IGBT, wherein a trench structure is formed from the front surface of a wafer, and gate electrodes are embedded in the trench structure via an oxide film, has increased, compared with a so-called planar IGBT wherein gate electrodes are provided in the front surface of a wafer.
Various improvement methods are proposed in order to reduce the on-voltage of the trench IGBT. For example, in a trench IGBT described in JP-A-2000-228519, after p-type base regions (p-type well regions) are formed in portions of the front surface layer of an n-type drift layer, the area ratio of the p-type base regions in contact with an emitter electrode is reduced on the emitter electrode side. Therefore, an outflow of holes injected from a p-type collector layer to the emitter electrode via the p-type base regions is suppressed, thus promoting the accumulation of holes in the front surface layer of the n-type drift layer. Further, the accumulation carrier concentration on the emitter electrode side is increased, thereby enhancing the conductivity modulation effect, and reducing the on-voltage.
However, in the case of the IGBT of JP-A-2000-228519, when holes flowing by turning on are accumulated in a region of the front surface layer of the n-type drift layer, in which no p-type base region is formed, there occurs a phenomenon wherein a displacement current flows to the gate electrodes due to the accumulated holes. As a result of this, there is the problem that a gate voltage rises sharply when turning on, and that the peak value of a collector current jumps with the sharp rise in the gate voltage.